This invention relates to processes for converting methanol and olefinic hydrocarbons to high octane liquid fuel. In particular, this invention relates to an integrated once through system for the production of methyl tertiary alkyl ethers in the presence of a high excess of methanol combined with the conversion of oxygenates and olefins to gasoline.
In recent years, a major technical challenge presented to the petroleum refining industry has been the requirement to establish alternate processes for manufacturing high octane gasoline in view of the regulated requirement to eliminate lead additives as octane enhancers as well as the development of more efficient, higher compression ratio gasoline engines requiring higher octane fuel. To meet these requirements the industry has developed non-lead octane boosters and has reformulated high octane gasoline to incorporate an increased fraction of aromatics. While these and other approaches will fully meet the technical requirements of regulations requiring elimination of gasoline lead additives and allow the industry to meet the burgeoning market demand for high octane gasoline, the economic impact on the cost of gasoline is significant. Accordingly, workers in the field have intensified their effort to discover new processes to manufacture the gasoline products required by the market place. One important focus of that research is a new process to produce high octane gasolines blended with lower aliphatic alkyl ethers as octane boosters and supplementary fuels. C.sub.5 -C.sub.7 methyl alkyl ethers, especially methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME) have been found particularly useful for enhancing gasoline octane. Therefore, improvements to the processes related to the production of these ethers are matters of high importance and substantial challenge to research workers in the petroleum refining arts.
It is known that isobutylene may be reacted with methanol over an acidic catalyst to provide methyl tertiary butyl ether (MTBE) and isoamylenes may be reacted with methanol over an acidic catalyst to produce tertiaryamyl methyl ether (TAME). In these etherification processes, a problem of major importance is the separation of methanol from the etherification reaction product due to the proclivity of methanol to form a very dilute azeotropic mixture with hydrocarbons and the strong solubility of methanol in both water and hydrocarbons. While it would be useful from an equilibrium standpoint to use large excesses of methanol in etherification, subsequent separation problems have limited that process improvement. Due largely to these factors, the cost associated with methanol separation and recycling in the etherification reaction represents approximately 30% of the cost of the total etherification process.
In U.S. Pat. No. 4,684,757 to Avidan et al., the well-known ability of zeolite type catalyst to convert methanol to olefins is utilized by directing unreacted methanol from an etherification reaction to a zeolite catalyzed conversion reaction for conversion to olefin, thereby obviating the need to separate and recycle methanol in the etherification reaction. However, the process of Avidan et al. converts oxygenate feedstock. The process incorporates an alkylation step in one embodiment to produce alkylate as well as C.sub.5 + gasoline and C.sub.5 + ethers.
The process for the conversion of methanol to olefins utilized in the Avidan et al. patent is but one in a series of analogous processes based upon the catalytic capabilities of zeolites. It is known that zeolites, such as ZSM-5, can convert methanol to hydrocarbons of higher average molecular weight. Depending on various conditions of space velocity, temperature and pressure methanol, and lower oxygenates in general, can be converted in the presence of zeolite type catalyst to olefins which may then oligomerize to provide gasoline or distillate or be converted further to produce aromatics.
The feasibility and adaptability of the basic chemistry of zeolite oxygenates conversion to produce useful conversion processes has been the subject of much inventive research activity. Recent developments in zeolite catalyst and hydrocarbon conversion processes have created interest in using oxygenates and olefinic feedstocks for producing C.sub.5 + gasoline, diesel fuel, etc. In addition to the basic work derived from ZSM-5 type zeolite catalyst, a number of discoveries have contributed to the development of a new industrial process. This process has significance as a safe, environmentally acceptable technique for utilizing feedstocks that contain lower olefins, especially C.sub.2 -C.sub.5 alkenes. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose converse of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al. have also contributed improved processing techniques in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above identified disclosures are incorporated herein by reference.
A well-known process for the conversion of oxygenates to gasoline is the methanol to gasoline process, known as MTG. The process is described in U.S. Pat. No. 3,931,349 to Kuo, U.S. Pat. No. 4,404,414 to Penick et al. and in the publication by C. D. Chang, Catal. Rev.-Sci. Eng., 25, 1 (1983). These references are incorporated herein in their entirety.
Recognizing the limiting problems of the etherification processes to produce MTBE and TAME and the potential that resides in the general area of the chemistry of oxygenate and olefin conversion with zeolites to resolve those problems, the following objectives of the instant invention have been established:
first, it is an object of the present invention to provide a once through integrated process for the production of liquid fuel mixtures from olefin containing feedstock and a high excess of lower alkyl alcohols by etherification and oxygenate and olefin conversion reactions. PA1 (a) contacting a first liquid reaction mixture in a single pass with an acid etherification catalyst under etherification conditions, said first reaction mixture comprising C.sub.4 -C.sub.9 hydrocarbons containing C.sub.4 -C.sub.7 isoalkene components and C.sub.6 + gasoline range non-etherifiable aliphatic components, and a lower aliphatic alcohol reactant, said alcohol being present in more than 30% stoichiometric excess of the isoalkene component; PA1 (b) recovering an etherification reaction effluent containing C.sub.5 + tertiary alkyl ether, gasoline range hydrocarbons, unreacted alcohol and light olefinic hydrocarbons; PA1 (c) distilling the etherification reaction effluent to provide a first product stream comprising a liquid mixture of C.sub.5 + ether and gasoline range hydrocarbons, and a second volatile low molecular weight reaction mixture comprising unreacted alcohol and light olefinic hydrocarbons; and PA1 (d) contacting the second reaction mixture with an acid medium pore metallosilicate zeolite catalyst at elevated temperature to convert the alcohol and light olefinic hydrocarbons to a second product stream having average molecular weight greater than the second reaction mixture.
It is another object of the present invention to provide a once through process for the production of liquid fuels of enhanced octane value containing MTBE and TAME.
A further object of the instant invention is an integrated liquid fuels process wherein the etherification reaction is conducted without the need to recycle excess alcohol to the etherification reaction.
A further object of the present invention is to provide a once through liquid hydrocarbon integrated process incorporating etherification with oxygenates and olefin conversion wherein valuable isoalkenes in the hydrocarbon feedstream are etherified prior to the conversion reaction.